| SETUP FLIGHT |
| PRELIMINARY SEXTANT CHECKOUT: 1. Test of installation and setup on runway With sextant active in main window: Use 'Go to airport' to get the game set up: Aircraft at Norfolk, Virginia. (KORF), runway 5 Note the location of the AC is 36*53N, 076*12W by using the Coordinates display on screen (for training purposes only). It is toggled by pressing Shift+Z. Set date/time to 14 March 1945, 0015 local time, 0515 Zulu Tune in the ADF on 329 Khz. The needle should swing to line up with the runway, 230 degrees. Its purpose is to help you visualize the line of position it provides, very similar to the LOP you will get from the sextant. Set Weather to clear On the sextant worksheet, set Assumed Position: 37*0' latitude, -076*0' longitude. **Don't forget the minus sign** Using Polaris for the star, set the Azimuth 0*0'; elevation 37*0' Using the UTC clock on the worksheet as a timer, click on the eyepiece hot-button. It should display the enlarged eyepiece for exactly 60 seconds. Click twice on the + bubble control to ensure you have a large bubble. Watch the bubble and star jitter, and note that, on average, the star is about 1/2 of the large bubble diameter below the center of the bubble. When the enlarged display extinguishes the display should show distance 0 nm. This says we measured our distance to be about 0 nm south of the assumed LOP. Because the star azimuth is true North, and the assumed Latitude is 37 degrees, this LOP is the 37th parallel of latitude. Start another 1 minute run by resetting and starting the sextant timer by clicking the eyepiece twice. During the run click the + fine elevation knob repeatedly but slowly 20 times. Notice the star is moving up the display (despite the jitter). This should move the star about halfway between the bubble and the top of the display, and leave it there till the light goes out. Now the distance should read about minus 100 NM since the measurement was deliberately miss-set. Next start another 1 minute run, and try to center the star in the bubble. Because for this training exercise we know our location is 7 NM south of the 37th parallel, keep your eye on the sextant elevation, and see if you like the sight picture when you are at 36*55' degrees of elevation. That is what you are looking for. Don't worry about the jitter. Don't chase the bubble! Make the corrections slowly over a few cycles, to end up at your best judgment. That is why real bubble sextants have averaging devices built in. They were not very accurate without them. Why 36*55'? 5 minutes of latitude is 5 NM, and your real location is further away from the star than your assumed position, so the angle will be smaller, and the distance will be negative on the worksheet. Your choices are -5 or -10 based on the coarseness of the fine elevation. -5 is the closest to the precise -7, which is the actual distance you are to the south of Latitude 37.0 North. If you happen to have the sectional chart for Washington, you can see this clearly on the chart. Otherwise, call up your FS map, and it shows the same information. When you are comfortable with the way the fine elevation knob works, let's take a look at another neat feature of the sextant. Suppose you were 6 degrees east of where you thought you were. Say, your assumed longitude was 81W instead of 076W. Set that change into the sextant, and rerun your position. Note that it still accurately puts you on the east-west line of position 5 nm south of the 37th parallel. But you still don't have a clue, yet, as to where you are on that line. This is where the second LOP comes in. Suppose that, to make it simple, a star due east of the airport was at 13 degrees above the horizon at the time you plan your shot. Lets plan a shot at 0530 hrs of this star. Reset your assumed latitude to 37N 076W. Set the star azimuth to 90, and elevation to 13 degrees. At 0529 hrs and 30 seconds, start your measurement. You should see the distance is -10 NM, which is to say that you are on a north/south line of position 10 NM west of the -076th longitudinal meridian. Check it with your ruler or dividers if you have a sectional chart, or eyeball it on your FS9 Map, or use the VE6B and enter the lat. and lon. of your true position and your assumed position, and see that 10 nm is pretty close (It shows 9.6 nm to the 37th parallel). So the intersection of these two lines of position, adjusted from the assumed position by the proper distances, is just where you expected them to be, near the aircraft. One last observation. What if our assumed position was at 081 W, like before? Set it up, N37, W081, and now that star you used would appear lower in the sky. You may still use it, or pick another one, on the same azimuth (for this demo) Assume that at 0530Z, the star is at 11 degrees (We have grossly simplified this problem, but there are of course look up tables for the complete answer). So, we set in an az of 90, and an el of 11, and take another shot. The distance is 230 nm. Let's see how far off we are. Using the VE6B, 37N and 081W for departure 37N and 076.2W for destination, the distance is 230 nm. The significance of this is that even if our assumed position is off almost 5 degrees in longitude, the sextant will properly locate the aircraft. In real solutions, the stars are not lined up so well, so the LOPs don't line up exactly with latitude and longitude lines, but the accuracy is the same. Just harder to visualize and to draw. And Polaris is unique in that it doesn't move around very much. 2. Test of sextant at cruise altitude So, let's get in the air from KORF, runway 5, turning due East (true), or 100 degrees magnetic at 500 ft AGL, turning on the autopilot, if desired, to hold your heading and climb to 2000 ft cruise altitude. Fly due East a while just south of the 37th parallel. Trim for cruise at 150 KTAS. The ADF needle should be swinging steadily toward 280 degrees, and if you take a shot of Polaris every four minutes using the original star and assumed position settings, you should see the distance remain at about -5 nm. Then decide to fly up to, but not beyond the 075th West meridian. This is of course before the days of DME, so a sextant is the gauge of choice. Set the Assumed position at 37N, 075W. Select the star to the east at 25 degrees elevation. Now, every four minutes, take a sextant reading, and note the distance going toward zero. You can quickly get the ground speed by dividing the change in distance by the time change. Then you can calculate the time when you will be at 0 distance, which corresponds to the 075th Meridian. Since this is about a 20 minute flight, you may want to go to 4x speed till you get close to the meridian. Continue to take readings (now on one minute real time intervals). You should start your standard rate turn one minute before getting to zero miles. After completing the turn and starting back on course 280 degrees magnetic, you can do the same type of shot and calculation, watching your increasing distance. Now, for instance, you could plan your SOD based on calculated position, ground speed, and sextant readings. Take a peek at the track on the map and congratulate yourself for the nice looking track. You are now navigating! 3. A simple practical example daytime flight Another example of sextant usage is a flight from the Naval Air Station, Ford Island, Oahu, Hawaii to Johnston Atol USA. NPS to PJON, 713 nm. The average magnetic bearing is 236 degrees, and you must use the compass, sextant and the Sun as the sole means of navigation. Using Elgen Long's rule that your final error (the distance between where your ded reckoning puts you and where you actually are) probably will not exceed one mile in ten, since the distance is 713 nm, you can expect to arrive in the vicinity of PJON with an error less than 71 nm. He means by this that 9 times in 10 you will arrive within a circle centered on PJON with a radius of 71 nm. This is called your circle of uncertainty, and unpredicted changes in wind strength or direction are a main contributor to it. Offset your heading to 242 degrees so your circle of uncertainty will lie just to the north of PJON. |
| This image depicts the first steps in the flight planning process. The red line is the FS Planner generated great circle route from Oahu to Johnston Island. The circle represents the 71 NM radius of uncertainty, and the black arrow represents the bearing offset to the north so that you can be sure you are to the north side of the island when you turn south to fly down the sun line of position. |
| figure 18 The images in this section are depictions only, and not suitable for computing angles, distances, or bearings. The distortion created by being north of the equator is not mapped properly. Next, look up the position of the SUN from PJON for the afternoon, when you expect to arrive. For 29 August 2004 the USNO says (approximately) : |
| Using this information, choose to arrive on the LOP at 0330 hrs. At 150 Knots Ground Speed, you should take off 4 hours and 45 minutes before that or at 22:45 GMT. This is 12:45 Hawaii local time, per Microsoft FS. |
| On your sextant worksheet, enter the az and el of PJON, which should remain your assumed position for the whole trip. N16*45', W169*30' (rounded off to the nearest 5 minutes). Take off, climb to cruise altitude, adjust your 242 magnetic heading for any needed cross wind compensation, and fly ded reckoning until 0130 hrs GMT when you should try to take your first daytime Sun shot. If you are beyond about 300 nm from the assumed LOP thru Johnston Island, the sextant will not give you to useful results. The elevation angle and distance will be blanked out either immediately, or at the end of the 1 minute shot average, and you may not see any stars or sun in the eyepiece. If you do get a reading, it will be useful in determining your course made good. By 0230 hrs GMT you should be able to make the shot, and record the distance. You can now make a good ded reckoning estimate as to when the distance from the line of position will be 15 nm east of PJON. Figure 19 below shows a chart of the destination portion of the flight. The added blue line thru the destination represents the sun line of position at 0230 GMT thru an assumed position at Johnston Island. It also shows how the LOP would be adjusted, if the sextant reading at that time showed an angle of 42*35'. This corresponds to a measured distance of 140 NM away from the sun LOP thru the destination. |
| figure 19 You may click on the "figure 19 " hotbutton to display and then print out a clean copy of this image, to plot your own LOP. Then use the browser's "back" button to return to this page. The resulting estimate of position is at the intersection of the right hand blue line, and the black line, at about N19, W167. The sun is on an azimuth of 267 degrees. Note that since you are not flying directly toward the sun, the distance you have to travel to reach the LOP is greater than 140 NM. Since you are flying 20 degrees left of the direction to the sun, using a bit of trig. you need to fly 150 NM, or exactly one hour at a Ground Speed of 150 Knots to reach the LOP. This is not surprising since the shot was timed to be 1 hour out. Interpolating the Sun data above (or use figure 20 as described below) to get the proper sun angle for a specified time, make another shot when you think you are 15 nm from the LOP, and based on this new speed line, make your turn toward PJON when your ded reckoning says you are on the LOP. The course you turn to will be the azimuth to the sun minus 90 degrees, so the bearing depends on your ETA at the LOP. |
| figure 20 Figure 20 depicts the way the navigation chart might look at 0324 GMT, if the sextant says the angle to the sun is 29*45' and the distance is 15 NM away from where the LOP would be if the plane were on the LOP thru the destination. The direction to the sun is now 271 degrees, and at 150 Knots Ground speed, the turn south on the sun line should be made at 0330 GMT. |
| This image was used in the example to determine the sun height as a function of time. It is used at Johnston island as follows: Select a time to make a sun shot. Find the corresponding sun elevation, and set that angle into the sextant. Start the sextant and make a measurement of the angle. The display should show the angle and the distance in NM. Move the blue LOP left or right to plot the measured LOP. Don't start your let-down, or make any unnecessary speed changes until after you turn toward PJON. Now you can continue to make Sun Shots on the way down, perhaps every 10 minutes, to stay on the line of position. Note that it will shift a few degrees to the right as the Sun gets lower in the sky. |
| figure 21 The general practice is to adjust your course by 30 degrees for the time you project it will take to get back on course, then come back to your course line. If the uncertainty is 71 nm, you may overfly the airport immediately after turning south, or you may have to wait up to 1 hour to search the 142 nm. It is a very good idea to take a long hard look to the north just before you turn south, because if you are already south of PJON when you make your turn, it is a long swim home! The idea is to just fly until the sextant says 0 distance to your LOP through the destination, and turn and fly down that line to the airport. |